High-speed interfaces such as a Universal Serial Bus (USB) and a High-Definition Multimedia Interface (HDMI) transmit signals that are approximately 180° out of phase with each other through a balanced line with a differential transmission method. In the differential transmission method, since radiation noise components or exogenous noise components cancel each other, they have little effect on the transmission of signals. However, in a case where the superimposition of common mode noise occurs between the ground and a balanced line or common mode noise is generated because of the asymmetry of a balanced signal, unnecessary radiation (EMI/EMS) occurs.
In order to suppress such common mode noise, a common mode choke coil is generally disposed on a balanced line.
For example, as disclosed in Japanese Unexamined Patent Application Publication No. 2008-098625, a common mode choke coil has a structure in which two choke coils having the same winding direction are coupled. Accordingly, when a common mode current flows, magnetic fluxes generated by the two choke coils are combined in a magnetic core, a large mutual inductance is generated, and the common mode current is reduced. That is, when a common mode current flows through a balanced line on which a common mode choke coil is disposed, an induced voltage is generated at the two choke coils in the direction in which the common mode current is canceled. As a result the common mode current is reduced.
In order to enhance the effect of reducing a common mode current (common mode noise), it is necessary to increase the degree of coupling between choke coils. Accordingly, two choke coils in a common mode choke coil are generally included in a magnetic substance.
In a high-speed interface using a balanced line, in order to achieve a high-speed transmission signal, the structure of an IC itself tends to be vulnerable to Electro-Static Discharge (ESD). Accordingly, an ESD protection element is often used. As disclosed in, for example, Japanese Unexamined Patent Application Publication Nos. 2009-065190 and 2010-141642, a composite device obtained by integrating a common mode choke coil and an ESD protection element has been developed.